Insights into heterogeneous streamflow generation processes and water contribution in forested headwaters
Understanding how diverse headwater streams contribute water downstream is critical for accurate modeling of seasonal flow dynamics in larger systems with changing climate. This study investigated how headwater catchments, with diverse subsurface architecture, seasonally influence downstream flows within Lookout Creek—a 62 km², 5th-order catchment in the rain-snow transition zone in western Oregon, USA. We analyzed one year of hydrometric and water stable isotope data collected at ten streams locations, alongside 10 years of precipitation isotopic data. As expected, isotopic data revealed that most of the streamflow was sourced from large fall and winter storms. Generally speaking, stream isotope ratios decrease with elevation. However, some streams were more enriched in heavy isotopes than expected, reflecting the influence of isotopically enriched storms and relatively high dynamic storage. Other streams that tended to have dampened flow dynamics were relatively depleted of heavy isotopes indicating higher elevation water sources than their topographic watershed boundaries. Both hydrometric data and water isotope-based member mixing models suggest variable dynamic and total storages that influence differences in seasonal water contributions from tributaries. Most notably, the contributions of Cold and Longer Creeks, which occupy less than 10% of the Lookout Creek drainage area, sustain up to 50% of the streamflow in the summer. These catchments have low dynamic storage and high groundwater contributions, as evidenced by shallow flow duration curves. Finally, our data suggest that geologic variability and geomorphic complexity (presence of earthflows and landslides), results in variable dynamic storage that dramatically influences the seasonal distribution of streamflow and the response of streams to precipitation events. Heterogeneity in headwater catchment architecture is key to understanding flow dynamics in mountainous regions; our study highlights its critical importance in mediating the response of streams to changes in climate and other disturbances.